1. Field of the Invention
The present invention relates to a method and apparatus for processing a color image. More particularly, the invention relates to a method which is capable of converting color signals in color coordinates of colors C (cyan), M (magenta), Y (yellow), K (black) to corresponding ones in another color coordinates while keeping invariable the calorimetric values and a black-ink ratio, and an apparatus for executing the method.
2. Discussion of the Related Art
A color management system (CMS) secures the consistency of colors in different color image input/output devices by properly managing colors. The CMS or a color facsimile converts different input color signals C, M, Y, K to calorimetric values, and converts again these calorimetric values to the output color signals C, M, Y, K. In this way, the calorimetric coincidence of the input color signals with the reproduced color signals is secured.
In the color signals C, M, Y, K, colors are expressed a four-dimensional manner, while the calorimetric values are expressed a three-dimensional manner. Accordingly, the calorimetric values can be uniquely determined by the input color signals C, M, Y, K, but the color signals C, M, Y, K cannot be uniquely determined by the calorimetric values. For this reason, the conventional art generates the color signals C, M, Y, K in the following manner.
Tentative color signals C, M, Y, K are uniquely determined on the basis of the calorimetric values. The minimum values of the color signals C, M, Y are selected from among those tentative color signals C, M, Y, K, and used as the maximum quantity of black ink or key (referred to as "black"). The maximum quantity of black is multiplied by a proper black ratio to thereby form a black print or determine a value of the signal K. Color signals C, M, Y, K that are calorimetrically equivalent to the input ones are generated by using the thus obtained signal K.
In reproducing black by using color signals C, M, Y, K, when a case where color signals C, M, Y are set at 100% and the signal K is 0% (process black) is compared with another case where color signals C, M, Y are set at 0% and the signal K is 100% (single color of black), the colorimetric values in the first case are substantially equal to those in the second case. However, other parameters, e.g., gloss, than the colorimetric values in the first case are different from those in the second case. For example, it is desirable to express a black character by a single color of black. However, it is undesirable to express a black part in a color photograph by a single color of black.
Such information at the time of color separation which are those other than the calorimetric values, are contained in the color signals C, M, Y, K. When the color signals are converted into colorimetric values as in the CMS, other information than the calorimetric values are lost. Thus, the conventional art cannot reproduce an image having the same texture as of the original image. The information at the time of color separation can be retained if gradation levels of each single color are corrected without converting the colors. In this case, it is impossible to form the output colors which are calorimetrically equal to the input colors, however.